What Is Computer Numerical Control (CNC)?

In manufacturing, parts are commonly created by removing material from a larger block of material (often called a “workpiece” or “stock”) using machines such as lathes, drills, or laser cutters. This production method is known as subtractive manufacturing because material is removed from the workpiece. Other manufacturing methods might add material (e.g., additive manufacturing) or use heat and molding techniques (e.g., injection molding, blow molding).

You may be familiar with subtractive shop tools such as hand drills and orbital sanders. More sophisticated manufacturers use tools that follow similar principles but are enhanced with computer controllers, feedback systems, and sensors to add precision, speed, and automation to the production process. The most advanced of these methods is found in computer numerical control (CNC) machines.

Computer numerical control refers to the use of computers to control machine tools in the manufacturing process. Programmers provide instructions to the computer, specifying which tools the machine should use and their position, movement, and speed. The machine then carries out the instruction, notably with little human interaction.

Since the instructions come from a computer program, parts can be mass produced with little supervision. Plus, the machine can be reprogrammed. A manufacturer can redeploy equipment for new projects or return to old programs and parts as needed.

Consider, for example, a shop that produces fan blades. It can easily adapt to special orders by simply changing the program. In the past, the shop might have had to create expensive tools and jigs to adapt to changing customer requirements. Now, changing part size might be as simple as changing a few numbers in the program.

CNC has advanced manufacturing in many ways:

• Speed: CNC systems can carry out complex operations more quickly than before. One standout example is multi-axis machining. CNC systems can work simultaneously on a part on several axes at once (think linear and rotational dimensions). Production of intricate parts with curved surfaces, undercuts, and varying angles can now be completely automated.
• Accuracy: CNC-enabled machines follow exact computer instructions. They can also integrate with other tools to increase accuracy. They can:
  • Pull dimensions directly from CAD models that have already been validated for optimal performance. Since the systems are interoperable, there is little danger of errors in translating data from one system to another.
  • Use sensors to provide in-process monitoring and quality control. The system can quickly catch, and even adapt to, excessive vibration or other abnormalities.
• Repeatability: As discussed above, a part can be reproduced easily using CNC. Note, too, that a program can be reused months or even years later. The machine does not require reprogramming if the manufacturer needs to replicate a legacy part.
• Fewer errors, less rework, and reduced waste: Consistency throughout the system and the absence of manual operation reduce the risk of errors. With CNC, companies can create highly dependable and precise manufacturing operations with fewer errors, less rework, and, ultimately, less waste.
• Lower costs, optimized labor: CNC eliminates the need for manual operators, so machinists can use their expertise for higher-level tasks, such as programming and setup, troubleshooting, and process optimization.

The basic steps for CNC production overlap with those for conventional machinery. Workers must inspect and calibrate the machinery; tools and materials must be loaded into the equipment; and part offsets adjusted.

But with CNC, programmers must set up the digital instructions for the machinery. They’ll often start with the product’s CAD design to indicate dimensions and coordinates. They may migrate the CAD data into a computer-aided manufacturing (CAM) system to produce toolpaths. In many cases, the CAM system can write the final code (called G code) for CNC operation.

Next, an operator loads the program into the system and then conducts a test run. Once proven, production begins.

It’s important to note that the CNC program is valuable intellectual property (IP). As such, most companies will further document and archive the code for traceability, quality control, troubleshooting, compliance, and training purposes.

If you work around legacy equipment, you may run into numerical control (NC)-driven equipment, which is similar but not the same as CNC. NC is an older technology that was also designed to automate manufacturing machinery, but it has many limitations not found in today’s CNC systems. Here are a few comparisons between NC and CNC:

NC systems get their instructions from punch cards or tapes. Their logic is literally wired into the equipment’s circuits. This makes NC machines difficult to reprogram, and they often perform only one task throughout their lifetime. To make matters worse, NC machines require highly skilled and difficult-to-find operators.

CNC machines, on the other hand, can be reprogrammed easily in the same way that you would reprogram any computer. The system requires no physical changes, and operations don’t require the highly specialized skills needed for NC. Plus, CNC programs can be reused, something not possible with NC.

Compared to CNC, NC can’t perform many complex operations. For example, an NC machine might produce simple brackets or drill holes into an engine block. But you wouldn’t expect it to handle intricate 3D curves. CNC systems, on the other hand, support multi-axis milling and can produce complex shapes.

NC machines tend to be less expensive to purchase, but they cost more to maintain compared to CNC systems, which tend to be pricey, but require less maintenance.

Given these limitations, it’s no surprise that NC machines are disappearing from the manufacturing floor. Newer CNC machines can do more with fewer headaches. Still, some older NC machines remain valuable and can still be found in operation.

Most subtractive-type machinery has been adapted with CNC today. You find computers in routers, pick-and-place equipment, and, of course, milling machines.

Examples of distinct and common CNC machines include:

You probably know that lathes turn material around a spindle while a cutting tool shapes the outside surface or bores a hole through the center. Shops have long used lathes like this to create cylindrical objects, such as wooden legs for a chair or table.

With the introduction of CNC technology, turning and tool movement takes place more quickly and accurately than ever before. CNC lathes perform complex tasks such as threading, contouring, and surfacing — all with the kind of precision needed for even the most demanding industries, such as aerospace and automotive. Plus, CNC machines can be set up to work seamlessly with equipment like bar loaders and gantries to automatically supply blanks to the spindle. The result is a highly automated process that requires just a fraction of the manual effort or skill required in the past.

A milling machine traditionally keeps the workpiece still while a drill or cutting tool moves across it. But today’s more sophisticated CNC capabilities can be set up to position and reposition the part on multiple axes. This allows for intricate cutting in almost any position.

As such, CNC milling machines can also produce highly complex geometries. Imagine a turbine blade whose design involves changing contours on multiple axes, something that cannot be easily created with a conventional, single-axis operation. Since CNC milling machines access every axis at any time, curved surfaces are produced quickly and accurately.

Drills, of course, are everywhere in manufacturing. They are necessary for providing locations for fasteners (think nuts and bolts), creating openings for airflow, and even removing weight from a chassis. A drill with CNC capabilities can automatically ream, counterbore, and tap holes at precise depths and in easily repeatable patterns, making it ideal for mass production.

Printed circuit boards (PCBs) provide a great example of CNC drilling work. With PCBs, very small fiberglass plates are packed with electronic components — most of which require carefully drilled mounting holes. A misplaced hole can lead to assembly problems and electronics failures. CNC drilling machines ensure boards are created to spec quickly and with low failure rates.

Grinding machines use abrasives and rotating wheels to smooth metal parts during production. CNC-enabled grinders automate the process and, again, ensure the parts are finished consistently to a specified smoothness. Because CNC grinders can work in micrometers, they can also be used to sharpen tools more accurately than conventional machinery, even those with complex geometries.

Laser cutters use powerful, focused CO2, neodymium (Nd), or Neodymium-doped yttrium-aluminum-garnet (Nd:YAG) lasers to cut or engrave materials. Since lasers can be focused down to a micrometer, they provide more accuracy than other cutting tools, and, therefore, produce less waste. Early laser cutters provided some controls and might have been used for custom acrylic signs or components. But with CNC technology added, operations are faster and more repeatable than ever.

Whether it’s aerospace, medical, automotive, or consumer products, CNC technology appears in almost every industry today. That’s because manufacturers all benefit from improved speed, accuracy, and flexibility. By automating and optimizing production, CNC systems create higher-quality products and quickly adapt to changing customer needs. And while legacy NC and manual machines can still be found in subtractive operations, it’s easy to see why CNC has become the clear winner in the race for superior manufacturing technology.

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